1. Field of the Art
The present invention relates to a mechanism control apparatus having a drum servo system therein, and more particularly to a circuit for stably driving a drum in a video tape recorder, even though a drum frequency generator signal (hereinafter, referred to as "DFG signal") dropps out.
2. Background of the Related Art
Generally, in a mechanism control apparatus employing a drum head, a DFG signal generated from the drum frequency generator is analyzed, and then drum motor driving is controlled according to the analysis result. In general, a video tape recorder (VTR) and a digital audio tape recorder (DAT) include the mechanism control apparatus and drum servo system for controlling stable rotation of drum heads. The drum servo system controls the rotation of the drum head in dependance upon the DFG signal output from a drum frequency generator.
FIG. 1 illustrates a block diagram of a known drum servo system used for a mechanism control system. The DFG signal, which is a pulse signal generated in accordance with the rotation of the drum head, is applied to a controller 102. As shown in FIG. 2, upon receipt of the DFG signal, the controller 102 resets a counter 103 at a point t1 where the DFG signal is falling down, and generates a preset signal PS shown in FIG. 1 for enabling the counter 103 at a point t2 shown in FIG. 2. Then, the counter 103 initializes a count value CNT which has been counted from the point t1 up to now, to start counting clock pulses CK, received from a clock pulse generator 101, beginning from the point t2. In other words, the counter 103 begins to count the clock pulses CK at the point t2, and continues the counting until the DFG signal reaches the next falling edge at time t3.
For example, in a VTR system, the clock pulse generator 101 receives a chrominance subcarrier signal fsc. Accordingly, it can be appreciated that the counter 103 counts the number of the chrominance subcarrier signals fsc generated for one period of the DFG signal. Further, the count value digital-to-analog converter 107.
C N T   f r o m   t h e   c o u n t e r   1 0 3   i s   s u p p l i e d ,   t h r o u g h   a n   o u t p u t   c i r c u i t   1 0 4 ,   t o   a
The controller 102 generates a sample signal SP at the falling edge of the DFG signal, i.e., at the point t1. A memory 105 is enabled by the sample signal SP to receive and store the count value CNT counted for one period of the DFG signal. A ROM 106 receives the count value CNT stored in the memory 105 to supply an output thereof to the digital-to-analog converter 107. The digital-to-analog converter 107 receives the outputs of the ROM 106 and the output circuit 104 to generate a drum motor driving voltage DV as shown in FIG. 3.
A voltage range for driving the drum motor is determined according to the count value CNT, and the digital-to-analog converter 107 converts the voltage range data into the drum motor driving voltage. The voltage range in a linear region D2 shown in FIG. 3 is established according to the data output of the ROM 106, and the voltage ranges in non-linear regions D1 and D3 are established according to the output of the output circuit 104. That is, the digital-to-analog converter 107 establishes the voltage range of the drum motor driving voltage DV in the linear region D2 according to the output of the ROM 106, and the voltage range in the non-linear regions D1 and D3 according to the output of the output circuit 104, so that if the rotating speed of the drum is normal, the drum motor driving voltage DV is established in the liner region D2. If the rotating speed of the drum motor is higher than the normal, the drum motor driving voltage DV lies in the non-linear region D3. If the rotating speed of the drum motor is lower than the normal, the drum motor driving voltage DV lies in the non-linear region D1. Thus, the digital-to-analog converter 107 generates a drum motor driving voltage DV higher than 1/2 V.sub.DD if the drum rotates slower than a reference rotating speed, and a drum motor driving voltage DV lower than 1/2 V.sub.DD if the drum rotates faster than the reference rotating speed. Consequently, the drum motor driving voltage DV is maintained at V.sub. DD /2.
In such a drum servo system, operation of the drum motor is controlled in dependance upon the DFG signal. Thus, if the DFG signal is dropped out, the digital-to- analog converter 107 generates a drum motor driving voltage DV of V.sub.DD (=5 V) in the non-linear region D1 owing to an abnormal count value CNT, which in turn causes a picture on the screen to flutter.